From d092f79489a00e3ccc2d7784e7a18e19aba4c35f Mon Sep 17 00:00:00 2001
From: Andriy Gapon <avg@FreeBSD.org>
Date: Tue, 12 Sep 2017 06:04:01 +0000
Subject: [PATCH] zfs_get_vfs: reference a requested filesystem instead of
 vfs_busy-ing it

The only consumer of zfs_get_vfs, zfs_unmount_snap, does not need
the filesystem to be busy, it just need a reference that it can pass
to dounmount.

Also, previously the code was racy as it unbusied the filesystem
before taking a reference on it.

Now the code should be simpler and safer.

MFC after:	2 weeks
Sponsored by:	Panzura
---
 .../opensolaris/uts/common/fs/zfs/zfs_ioctl.c    | 16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

diff --git a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_ioctl.c b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_ioctl.c
index 425aed567404..156c55c59e7a 100644
--- a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_ioctl.c
+++ b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_ioctl.c
@@ -3049,13 +3049,11 @@ zfs_get_vfs(const char *resource)
 	mtx_lock(&mountlist_mtx);
 	TAILQ_FOREACH(vfsp, &mountlist, mnt_list) {
 		if (strcmp(refstr_value(vfsp->vfs_resource), resource) == 0) {
-			if (vfs_busy(vfsp, MBF_MNTLSTLOCK) != 0)
-				vfsp = NULL;
+			vfs_ref(vfsp);
 			break;
 		}
 	}
-	if (vfsp == NULL)
-		mtx_unlock(&mountlist_mtx);
+	mtx_unlock(&mountlist_mtx);
 	return (vfsp);
 }
 
@@ -3545,7 +3543,9 @@ zfs_unmount_snap(const char *snapname)
 {
 	vfs_t *vfsp;
 	zfsvfs_t *zfsvfs;
+#ifdef illumos
 	int err;
+#endif
 
 	if (strchr(snapname, '@') == NULL)
 		return (0);
@@ -3557,23 +3557,19 @@ zfs_unmount_snap(const char *snapname)
 	zfsvfs = vfsp->vfs_data;
 	ASSERT(!dsl_pool_config_held(dmu_objset_pool(zfsvfs->z_os)));
 
-	err = vn_vfswlock(vfsp->vfs_vnodecovered);
 #ifdef illumos
+	err = vn_vfswlock(vfsp->vfs_vnodecovered);
 	VFS_RELE(vfsp);
-#else
-	vfs_unbusy(vfsp);
-#endif
 	if (err != 0)
 		return (SET_ERROR(err));
+#endif
 
 	/*
 	 * Always force the unmount for snapshots.
 	 */
-
 #ifdef illumos
 	(void) dounmount(vfsp, MS_FORCE, kcred);
 #else
-	vfs_ref(vfsp);
 	(void) dounmount(vfsp, MS_FORCE, curthread);
 #endif
 	return (0);